1. Field of the Invention
The present invention relates to a machine tool finger chuck having a centering device for centering a workpiece and a plurality of jaws for holding an outer peripheral portion of the workpiece, and in particular, to a technology for improving the mechanical rigidity of a chuck main body.
2. Description of the Related Art
In a conventional finger chuck, a plate-shaped front wall is disposed in front of a chuck main body. A centering device that centers a workpiece is disposed at a center portion of the front wall. A plurality of jaws that hold the workpiece are disposed in an outer peripheral portion of the front wall. A drawing member opens and closes the jaws through a spider and a rod. In the conventional finger chuck, the entire centering device protrudes from the front surface of the front wall. Thus, the distance from the front surface of the chuck main body to the workpiece mounting surface becomes long, thereby lowering the mechanical rigidity of the chuck main body. When the chuck main body rotates at high speed, a workpiece tends to vibrate, thereby lowering machining surface accuracy. A related art reference, for example, Japanese Utility Model Publication No. 64-3606 published Jan. 31, 1989 teaches a chuck main body with a centering device disposed in a cavity thereof. In this case, since the axial thickness of the chuck main body is large, it is heavy. Thus, the chuck main body cannot rotate at high speed, thereby lowering the throughput.
The conventional finger chuck can hold a workpiece with different wall thicknesses. However, while the chuck main body is holding such a workpiece, the spider is inclined and rotates about the center of an inner spherical surface thereof. In this case, the center of the outer spherical surface, namely the position of the center of gravity, radially moves corresponding to the inclination of the spider.
The product of the amount of movement of the spider and the weight thereof produces a dynamic imbalance in the finger chuck that rotates. The unbalance amount of the finger chuck results in a mechanical vibration thereof. Thus, when a workpiece with a large outer diameter, for example an aluminum wheel, is machined at a high speed, the machining accuracy lowers and the service life of the spindle bearing shortens. Consequently, the movement of the center of gravity cannot be ignored.